1. Field of the Invention
The present invention relates to a liquid crystal display device and particularly to a multi-domain vertical alignment liquid crystal display where a columnar spacer is provided on a wiring part.
2. Description of the Related Art
A liquid crystal display device has various features of the thin type, lightness, and low power consumption, or the like. The liquid crystal display devices have been widely used as not only the display monitor for a personal computer, a word processor, office automation (OA) equipment, personal digital assistance (PDA), and an in-vehicle navigation but also a display device of the LCD TV, and the like.
The display operation mode of a liquid crystal device, which is currently used most, is a normally white mode using a twisted nematic (TN) liquid crystal.
The liquid crystal display includes electrodes which are respectively formed on the opposed faces of two sheets of glass substrates placed opposite to each other, and alignment films formed on the both electrodes. The two alignment films are processed for aligning in the directions perpendicular to each other by means of rubbing and the like. On the outer face of each substrate, a polarization plate which is adjusted with the polarization axis is disposed so as to be in parallel with the rubbing direction of the alignment film on the respective substrates.
When a nematic liquid crystal with positive dielectric anisotropy is inserted in between these substrates, liquid crystal molecules contacting with the alignment film are aligned along the rubbing direction, and the alignment directions of the liquid crystal molecules on the both substrates are orthogonal. The liquid crystal molecules between the both substrates, then, align in a direction perpendicular to the faces of the substrates, as they rotate their alignment directions in sequence within a plane in parallel to the faces of the substrates.
Subsequently, the liquid crystals are aligned between the substrates with the angle of 90 degrees twisted.
For a TN liquid crystal display device having such a structure, light entering at the face of one substrate passes through a polarization plate, and enters a liquid crystal layer. When passing through the liquid crystal layer, the direction of polarized light is rotated by 90 degrees along the twist of the liquid crystal molecules, and the light passes through a polarization plate of the other substrate having the polarization axis which is perpendicular to that of the one substrate. This produces a display in a brightness state when no voltage is applied.
When a voltage is applied to common electrodes, a long axis of the nematic liquid crystal molecules with positive dielectric anisotropy is aligned in a direction perpendicular to a substrate face, hence eliminating the twist. For the straight polarized light which enters the liquid crystal layer in this state, the liquid crystal molecules do not exhibit refractive index anisotropy, whereby the polarizing direction of the entered light does not change. As a result, the light cannot pass through the other polarization plate. This produces a light in a darkness state when a predetermined maximum volt is applied. When putting back into a state where no voltage is applied, a display in a brightness state is back due to an alignment regulation force. Displaying in gray scale is made possible by controlling the inclinations of the liquid crystal molecules while varying the applying voltage, and thereby changing transmission optical power from the other polarization plate.
A TN-TFT liquid crystal display device is a display device of an active matrix type where a thin film transistor (TFT) is provided on each picture element as a switching element to control an applying voltage between common electrodes for every picture element. Since the TN-TFT liquid crystal display device is thin and light, and is provided with a large screen and a high resolution, it has been widely used for a display monitor of a personal computer, a portable television, or the like.
While enhancement of display quality is expected in such applications, the TN-TFT liquid crystal display device has a problem where an angular field of view provided thereby is narrow. As a technology for solving a problem related to the characteristic of an angular field of view of the TN liquid crystal display device, a vertical alignment (VA) mode liquid crystal has drawn attention. In the VA mode liquid crystal, in comparison with TN mode liquid crystal, which has been adopted in a number of liquid crystal display devices, a display quality in black is high, and alignment processing such rubbing is not necessary. Among the VA modes, a multi-domain vertical alignment (MVA) mode liquid crystal display device (hereinafter, referred to as MVA liquid crystal display device) has especially drawn attention because it can achieve a wide angular field of view. The MVA liquid crystal display device has not only been applied to a transmission liquid crystal display, but is also beginning to be applied to a reflection liquid crystal display device and a semi-transmission liquid crystal display device.
Various technologies have been disclosed with respect to alignment control technologies for an MVA liquid crystal display device. In Japanese Patent Laid-open No. 2001-249340 (hereinafter, referred to as Patent Literature 1), there is disclosed an MVA liquid crystal display device in which a singular point (+1 or −1) formed in an alignment vector field for liquid crystal molecules is controlled, and decreasing of transmissivity is curbed, thereby improving a response characteristic. This singular point is defined as a point where, among liquid crystal molecules, one in the middle is vertically aligned, while the other liquid crystal molecules are fallen down and aligned. When vertically seeing a liquid crystal layer from above, a singular point where liquid crystal molecules around the singularity fall down toward the singular point or draw away therefrom is called a singular point of +1 (hereinafter, referred to as +1 singular point) in an alignment vector field for the liquid crystal molecules. Meanwhile, a singular point where some of liquid crystal molecules around the singular point fall down toward the ssingular point and the other draw away therefrom is called a singular point of −1 (hereinafter, referred to as −1 singular point) in an alignment vector field for the liquid crystal molecules.
FIG. 1 is a plan view of a face of a liquid crystal display device for explaining a prior art on an alignment control technology in Patent Literature 1. FIGS. 2, 3, and 4 are sectional views taken along the lines I-I, II-II, and III-III, respectively. As shown in FIG. 1, at a +1 singular point 10, a liquid crystal molecule 6 is aligned toward a point, while, at a −1 singular point 11, some of the molecules are oriented in directions different from a point. Referring FIGS. 2 to 4, on the opposed faces of a lower substrate 20 and a upper substrate 21, both of which are placed opposite to each other with a predetermined space therebetween, a picture electrode 3 and a common electrode 7 are formed, respectively. On the picture electrode 3 and the common electrode 7, vertical alignment films (not shown) are formed. Between the upper and lower substrates, liquid crystal molecules 6 with negative dielectric anisotropy are inserted. On the picture electrode 3 and the common electrode 7, a protruding structure 9 and a protruding structure 8 are respectively formed as control parts on singular points for the alignment of liquid crystal molecules. The protruding structure 8 has a cross-shape when it is seen in a direction perpendicular to the face of the substrate. The protruding structures 8 are disposed in such a manner that the cross-shaped parts are adjacent one another at predetermined intervals. When voltages are applied to the picture electrode 3 and the common electrode 7 in such a liquid crystal display, a distortion is formed in an electric field where voltages applied through the protruding structure 8 and the protruding structure 9. Because of this distortion in the electric field, the alignment of liquid crystals is restricted on the protruding structure 8, the protruding structure 9, and in the vicinity thereof. As shown in FIG. 1, in the middles of the protruding structure 8 and the protruding structure 9, +1 singular point 10 are formed, while −1 singular point 11 are formed at ends of the protruding structure 8.
In Japanese Patent Laid-open No. 2001-264773 (hereinafter, referred to as Patent Literature 2), there is disclosed an MVA liquid crystal display device where non-uniformity in a gap between the substrates is reduced, and thereby a display quality is improved. In this liquid crystal display device, as shown in FIG. 5, for a pair of substrates which are placed opposite for achieving a multi-domain, a plurality of waveform protrusions 22 are formed on at least one of the opposed faces of the pair of substrates. A waveform protrusion 22 has a continuous triangular waveform in one direction. In a second direction crossing the above direction, a plurality of columnar spacers 24 are disposed. The columnar spacers 24 work as keeping the distance between the pair of substrates to be constant, and are disposed on a shaft passing angles of the waveform protrusions 22. A configuration designed in such a manner is expected to be able to prevent occurrence of non-uniformity in gap. The waveform protrusions are contained in a concept of a singular point control part for the liquid crystal molecules alignment. In FIG. 5, reference numeral 23 indicates a picture element formed on a color filter (not shown).
In recent years, applying of an MVA liquid crystal display device to a mobile phone and a personal digital assistance has been considered. In such an application, a transparent touch panel may be provided for inputting information by pressing a liquid crystal panel with a finger or a pen point.
As in Patent Literature 2, when the waveform protrusion 22 is formed of a triangular waveform, singular points may be placed anywhere on a line of the waveform protrusion 22, and are the positions thereof are not to be determined at once. So long as it is on the line of the protrusion, electric potential is the same, and thus, there is no specific location in terms of electric potential. Since liquid crystal molecules are successively aligned, unless the positions of the singular points for the alignment of liquid crystal molecules are determined, the alignment of liquid crystal molecules in another region will not be determined. Therefore, it requires a certain period of time until the alignment of liquid crystal molecules in a picture element comes to a final state. In Patent Literature 2, there arises a problem where, when a panel surface of a liquid crystal display device is depressed, a wavy alignment irregularity occurs in liquid crystal molecules, and it takes a time in the order of seconds until restoring.
Furthermore, in the prior art of Patent Literature 1, singularities for the alignment of liquid crystal molecules, the singularities occurring on a signal line, are controlled by thickening or thinning a part of an electrode used for a signal line. The position of the singular point for the alignment of the liquid crystal molecule is influenced easily by the difference of the pattern formation accuracy when the signal line is formed in such a method. As a result, a parasitic capacitance between a signal line and a picture electrode tends to vary for every picture element, and thereby a flicker tends to occur. In addition, since the columnar spacers are not provided, a change in gap occurring when a panel surface is depressed is large, thus causing a wavy alignment irregularity in the liquid crystal molecules.